This invention relates to digital packet telecommunications, and more specifically to methods for minimizing retransmissions based upon automatically determined characteristics of the underlying network connection.
The TCP/IP protocol suite is employed in the Internet, a world-wide data communication network environment, and is also employed in local networks called Intranets. TCP technology intentionally omits explicit supervisory function over the rate of data transport over the various media comprising the network. While there are certain perceived advantages, this characteristic has the consequence of juxtaposing very high-speed packets and very low-speed packets in potential conflict, which results in inefficiencies. Resulting problems of latency and bandwidth constraints manifest themselves in delay, lost information and unsatisfied users.
The Transmission Control Protocol (TCP) provides a reliable session-based service for delivery of sequenced packets of information across the Internet. One method employed to provide reliability is to expect each end to acknowledge the data received from the other end. However, data segments and acknowledgments can be lost. To solve this problem, TCP sets a time-out when it sends data, so that the data unacknowledged is retransmitted. The TCP protocol must therefore manage a retransmission timer for each connection. The retransmission timer is used whenever TCP expects an acknowledgment from the other side. Upon transmitting some data, TCP tracks a retransmission time-out (RTO) value and a round trip time (RTT) for the given connection. If an acknowledgment is not received by the time the RTO expires, TCP retransmits the data using an exponentially increasing value for the RTO. This doubling is called an "exponential back-off." The RTO is computed as a linear function of the RTT, which is continuously tracked by TCP for each connection. Its value changes over time with changes in routing and network traffic load.
Implementations of TCP may have either hard coded values for the initial minimum RTO and initial RTO values for each connection, or alternatively, a single set of RTO parameters to be applied to all connections. However, operating system configurations may be unable to provide reasonable RTO related parameters for individual connections due to disparities in speed among connections.
For example, the default setting for RTT in the Solaris version 2.5 operating system is 500 milliseconds, and the default minimum RTO value is 200 milliseconds. By contrast, a previous generation of SUN operating systems (Sun OS) employed a default initial RTT value of 1,500 milliseconds. Thus, when Solaris 2.5 is used in web server applications, its RTT default values lead to excessive gratuitous retransmissions for web traffic from remote browsers with lower speed network access.
There is a natural variance in RTT based on the serialization time of a maximum size segment PCP data packet, especially at the start of a transmission. As a transmission window opens, the time for the acknowledgment of a data packet must incorporate the serialization time of the preceding packet as well as its own.
One technique for dealing with such a problem is to set a large default value for RTO. But this is inefficient when used over high speed serial lines (for example, T1) because timeout detection is delayed. For example, if an RTT is 200 milliseconds, it is inefficient to establish a minimum value RTO of 2,000 milliseconds because it will take on the order of ten times the RTT for an RTO to occur. However, if the default parameters for RTO are set too low, then gratuitous retransmission occurs on virtually every low speed connection.
In a copending U.S. patent application Ser. No. 08/742,994, now pending, in the name of Robert L. Packer, entitled "Method for Explicit Data Rate Control in a Packet Communication Environment Without Data Rate Supervision," a technique for automatically scheduling TCP packets for transmission is disclosed. Furthermore, in a copending U.S. patent application Ser. No. 08/762,828, now U.S. Pat. No. 5,802,106, in the name of Robert L. Packer, entitled "Method for Rapid Data Rate Detection in a Packet Communication Environment Without Data Rate Supervision," a technique for automatically determining the data rate of a TCP connection is disclosed. While these patent applications teach methods for solving problems associated with scheduling transmissions and for automatically determining a data flow rate on a TCP connection, respectively, there is no teaching in the prior art of methods for explicitly managing retransmissions of TCP packets based upon a minimum timeout value.
What is needed is a method for managing retransmissions to minimize the number of gratuitous retransmissions that take place on TCP connection based upon automatically determined characteristics of the underlying network connection.